The invention relates to a method for automatically switching off an internal combustion engine.
Currently, internal combustion engines are usually switched off manually by the driver by means of an ignition key. An exception is introduced in German Patent Document DE 100 23 331 A1. In this case, the brake pedal position or the brake pedal pressure is analyzed instead of the ignition key. The switch-off operation of the internal combustion engine will be initiated if, after the motor vehicle has been braked to stop, the brake pedal is increasingly actuated in its already actuated position. This type of a method has the disadvantage that only the motor vehicle driver is responsible for switching off the internal-combustion engine. Analyses of the current driving behavior have demonstrated that, despite a raised awareness of the environment and higher fuel prices, a manual switching-off of the internal combustion engine, for example, at traffic lights, rarely takes place independently.
In order to reduce fuel consumption and pollutant emissions, methods and systems are currently being developed and partially also already in use, which automatically switch off the internal combustion engine of a motor vehicle under certain prerequisites or in the presence of specified switch-off conditions and automatically switch the internal combustion engine back on in the presence of specified switch-on conditions. Such methods and systems or start/stop devices are mainly suitable for city traffic for reducing fuel consumption because, in city traffic, the vehicle often comes to a stop at traffic lights or as a result of traffic, and the operation of the internal combustion engine is not required.
A system is known, for example, from European Patent Document EP 0 158 605 B1, which causes a switching-off or restarting of the internal-combustion engine as a function of different conditions. One condition for initiating an automatic switch-off operation is, for example, that the vehicle speed has to be below a threshold value for a certain time before the internal combustion engine will automatically be switched off.
From German Patent Document DE 10 2008 060 350 A1, a method for automatically switching off an internal combustion engine of an automatic vehicle by means of a start/stop device is known, by which the start/stop device automatically switches off the internal-combustion engine when the vehicle has been braked to a stop and was held in the stopped position for a specified time interval by an actuation of the brake pedal.
In the case of vehicles with automatic transmissions which are automatically switched off when the vehicle is held in a stopped position, as a result of the fact that the transmission, or the transmission-side turbine of a torque converter arranged between the engine and the transmission, is stationary and the engine or the engine-side pump of the torque converter non-rotatably connected with the engine, is still running, the problem arises that, because of the rotational-speed difference between the engine output (or impeller of the converter) and the transmission (or output rotor of the converter), a transmission absorption torque will build up which, during the automatic switch-off of the internal-combustion engine, may result in a jolt during the engine coast-down because of longitudinal and transverse vibrations, which may be perceived as uncomfortable by the driver.
For reasons of completeness, reference is also made to German Patent Document DE 10 2007 009 833 B4, which discloses a method for controlling an automatic switch-off operation of an internal combustion engine, by which the switch-off measures are divided into preparatory switch-off measures and remaining switch-off measures. The preparatory switch-off measures for automatically switching off the internal combustion engine are carried out even before the occurrence of all specified switch-off conditions, so that a fast and more comfortable automatic switching-off of the internal combustion engine becomes possible.
It is now an object of the invention to indicate a method for automatically switching off an internal-combustion engine in a vehicle, which permits a comfortable automatic switching-off of the internal-combustion engine.
According to one embodiment of the invention, this object is achieved by a method for automatically switching off an internal combustion engine in a motor vehicle, preferably in a motor vehicle with an automatic transmission, by means of a start/stop device, initiates measures for automatically switching off the internal combustion engine if (in addition to the other specified and met switch-off conditions) the vehicle has been braked to a stop and advantageously held at the stop by an actuation of the brake pedal for a specified time interval. Within the specified time interval after having reached the stop, the driver can be given the opportunity to prevent the automatic switching-off. The automatic switching-off should, for example, be prevented if the driver recognizes that a continuation of the drive is imminent.
It is now the basic idea of the invention to ensure by appropriate measures that a low transmission absorption torque is present when the automatic switch-off operation is initiated, so that, when the internal combustion engine is automatically switched off, either no jolt or only a very slight jolt is generated as a result of the rapid torque reduction. Based on the above, the method according to the invention is characterized in that, at a defined point in time before the initiation of the measures for automatically switching off the internal combustion engine, the rotational speed of the internal-combustion engine is changed, or advantageously reduced.
As a result of the reduction of the rotational speed of the internal-combustion engine, the differential speed between the (rotating engine) and the (stationary) transmission is also reduced, so that, in turn, a reduced transmission absorption torque will occur. Thus, during the automatic switch-off operation of the internal-combustion engine, only a lower transmission absorption torque has to be reduced, which, in turn, causes lower longitudinal and/or transverse vibrations. The possibly still occurring jolt will become smaller and advantageously so small that it will no longer be perceived as uncomfortable by the driver. The defined point in time may be specified to be fixed or variable.
Advantageously, the rotational speed of the internal-combustion engine may be changed as a function of the actual rotational speed of the internal combustion engine and/or of the presence of specified operating conditions of the motor vehicle. Should the rotational speed of the internal combustion engine, for example, fall below an already low rotational speed threshold, so that a further reduction would result in a deterioration of the running of the engine, no further reduction of the engine rotational speed will take place. Likewise, it is meaningful to carry out the change of that rotational speed of the internal combustion engine only when specified operating conditions of the motor vehicle are present, which ensures that a change or reduction of the rotational speed of the internal combustion engine has no negative effects on the operation of the motor vehicle or on the automatic switch-off operation.
Advantageously, the amount of change of the rotational speed, particularly the amount of the reduction of the rotational speed of the internal combustion engine, may also be carried out as a function of the actual rotational speed of the internal-combustion engine and/or of the presence of specified operating conditions of the motor vehicle. For example, if the actual rotational speed is very high because of many active energy consuming devices of the vehicle, a higher reduction of the rotational speed of the internal combustion engine can be carried out than if the rotational speed of the internal combustion engine already is very low anyhow.
In a particularly advantageous further development of the invention, for example, the operation of an air-conditioning system of the motor vehicle may be taken into account because, during the operation of the air-conditioning system, the rotational speed of the internal combustion engine is higher, as a rule, than when the air-conditioning system is not in operation and therefore deactivated. The method according to the invention can therefore be further developed such that, during the operation of an air-conditioning system of the motor vehicle, which demands a higher rotational speed of the internal-combustion engine, at the defined point in time before the initiating of the measures for automatically switching off the internal combustion engine, the rotational speed of the internal combustion engine is reduced to a specified value, particularly to a value which is set when the air-conditioning system is deactivated, while the operating conditions of the motor vehicle are otherwise the same.
In addition to changing the rotational speed of the internal combustion engine at a defined point in time before initiating the measures for automatically switching off the internal-combustion engine, further preparatory measures can also be initiated, particularly those which also have a positive effect on comfort during the automatic switching-off of the internal combustion engine during the stopping of the motor vehicle equipped with an automatic transmission or automated transmission.
Advantageously, the method may therefore be further developed such that, at a defined second point in time before the initiating of the measures for automatically switching off the internal combustion engine,—in which case the defined second point in time may be identical with the defined (first) point in time)—a clutch of the automatic transmission is triggered in a changed fashion, particularly such that the clutch pressure is reduced. As a result, a slip will occur in the clutch, by which the transmission absorption torque can be reduced.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.